1. Field of Invention
This invention relates to a semiconductor process, and more particularly relates to a method of forming a strain-causing layer for MOS transistors and a process for fabricating strained MOS transistors.
2. Description of Related Art
As the linewidth of semiconductor device is much reduced and the requirement on the device speed increasingly gets higher, the strained-silicon technology is provided. In a metal-oxide-semiconductor (MOS) process utilizing the strained-silicon technology in the prior art, a stressed film is formed over the MOS transistors to cause strains in the channels regions thereof.
FIG. 1 illustrates a conventional stressed film for MOS transistors, wherein each transistor is based on a semiconductor substrate 100 and includes a gate structure 110. When the MOS transistors are NMOS transistors with n-type S/D regions 105, a tensile-stressed film 120 is formed over them. When the transistors are PMOS transistors with p-type S/D regions 105′, a compressive-stressed film 120′ is formed over them.
However, because the conventional stressed film 120/120′ is connected between two neighboring gate structures 110, the stresses for the two transistors counteract each other thereat, as indicated by the pair of opposite arrows, so that the strain caused is reduced and the performances of the transistors are lowered. This issue is important especially when the distance between the two neighboring gate structures 110 is small.